TagPollution Control Hearings Board(1532)

This document presents a statistical analysis of ambient water conditions and effluent discharge at a location designated 3RWN, examining the joint probability of stream flow and discharge occurring simultaneously. It includes a 3D probability surface chart, a data table showing dilution factors ranging from 3.6 to 5,294 at various probability levels, and a distribution curve of ambient stream discharge measured in cubic feet per second. The analysis notes that effluent is present approximately 38% of the time, with the 7Q10 flow of 0.16 cfs corroborating the 1st percentile and the 5Q30 flow of 0.15 cfs corroborating the 0.9th percentile.

Exhibit 2141 was not present at time of scanning. Administrative Record number AR 034564.02. No substantive content available; placeholder page only.

This Washington State administrative regulation establishes rules for 'mixing zones,' which are designated areas in bodies of water where pollutant discharges are allowed to mix and dilute before meeting water quality standards. The rules set strict size limits for mixing zones in rivers, estuaries, lakes, and ocean waters, and require that dischargers apply all available pollution-reduction technology before a mixing zone is approved. Special provisions are also included for storm water discharges and combined sewer overflows.

This technical document from a March 25 meeting discusses stormwater mixing zone analysis for proposed outfalls around Seattle-Tacoma International Airport (STIA). Because standard stream flow conditions (7Q10 and 30Q5) are extremely unlikely to coincide with stormwater discharge events, Parametrix proposed a site-specific approach using higher-probability rainfall-based discharge combinations instead. The analysis found that reasonable dilution of 14- to 24-fold could occur within about 15 to 20 meters downstream, though complete mixing requirements would exceed standard regulatory width and flow percentage criteria, prompting a request for exceptions under WAC 173-201A-100.

Handwritten notes outline eight key discussion points related to a mixing zone (MZ) study, including an examination of stream dynamics, regulatory questions about MZ implications under NPDES permitting, and how to weigh evidence for defensibility. The notes raise questions about whether mixing zones are appropriate at all, and whether existing regulations are flexible enough to accommodate site-specific MZ conditions. A final point flags that mixing zones near water quality listing designations will be a particularly difficult policy 'sell.'

This document outlines regulatory problems and compliance points for stormwater discharges into Washington State surface waters, focusing on where discharges must meet water quality standards. It identifies four key unresolved issues, including the lack of an approved design storm for mixing zones and undefined critical receiving water conditions. Various example situations are provided to help determine the exact location where stormwater discharge compliance must be measured.

This technical document describes a mixing zone analysis for stormwater discharge from the 3rd Runway North at Sea-Tac Airport into Miller Creek, designated as Basin SDW-1. The analysis uses design storm modeling with various rainfall scenarios (2-year/2-hour, 2-year/72-hour, and 3-year/72-hour events) to calculate effluent flow rates for both acute and chronic water quality criteria. Stormwater from the basin's approximately 128 acres of pervious and impervious area is routed through a detention pond before discharge, with outflow rates determined by a storage-discharge rating curve.

Exhibit 2084 from Administrative Record AR 033383.01 was not present at the time of scanning. This is a placeholder document indicating a missing exhibit. No substantive content is available.

This document, identified as AR 024333, has been officially withdrawn and contains no substantive content available for review. It is part of the administrative record but has been removed from public access, leaving only a placeholder page indicating its withdrawn status.

Letter and memorandum from EPA Assistant Administrator Robert Perciasepe transmitting the final Interim Permitting Approach for Water Quality-Based Effluent Limitations in Storm Water Permits to State Water Program Directors and EPA Water Management Division Directors. The policy addresses NPDES storm water permit effluent limitations, adopting best management practices (BMPs) in lieu of numeric water quality-based